It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk either removes the marker from the article, or deactivates the marker by using a deactivation device provided to deactivate the marker.
Known deactivation devices include one or more coils that are energizable to generate a magnetic field of sufficient amplitude to render the marker inactive. One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized in a certain manner, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency. The detection equipment used with this type of marker generates the interrogation signal and then detects the resonance of the marker induced by the interrogation signal. According to one known technique for deactivating magnetomechanical markers, the bias element is degaussed by exposing the bias element to an alternating magnetic field that has an initial magnitude that is greater than the coercivity of the bias element, and then decays to zero. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined interrogation signal frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
One challenge faced in designing marker deactivation devices is the need to provide reliable deactivation of a marker regardless of the orientation of the marker at the time that the marker is presented for deactivation. Co-pending patent application Ser. No. 09/016,175, filed Jan. 30, 1998 discloses deactivation devices in which two or more coils are wound around magnetic cores. The devices are rapidly switched between two modes of operation, including a first mode in which one of the coils is driven with an alternating excitation signal and the second coil is not driven, and a second mode in which the second coil is driven with the excitation signal and the first coil is not driven. The first and second coils are disposed with orientations that are mutually orthogonal, so that, considering both modes, a marker presented to the deactivation device experiences a substantial alternating field regardless of the orientation of the marker. In practice, the marker is swept past the deactivation device and therefore is exposed to the decaying alternating field required to degauss the bias element of the marker.
The above-referenced '175 patent application has a common assignee and a common inventor with the present application. The disclosure of the '175 application is incorporated herein by reference.
In designing the deactivation device having core-wound coils as disclosed in the '175 application, it was desirable to provide an energizing circuit to provide the rapid switching between the two modes of operation described above, while also operating efficiently. A significant element of efficient operation is high throughput; that is, the deactivation device should be able to deactivate a number of markers in rapid succession. A limiting factor in terms of throughput is the maximum speed at which markers can be swept over the deactivation device while still providing reliable deactivation. It is desirable that a deactivation device perform reliably even when a marker is swept quite rapidly over the device.
Another problem encountered in prior art marker deactivation devices relates to a detection circuit included in the deactivation device to detect the marker and then trigger generation of the deactivation signal field. If a marker presented for deactivation has a marker signal frequency that deviates from the nominal marker signal frequency, the detection circuit may fail to detect the marker, so that operation of the deactivation device is not triggered, and deactivation does not occur. As a result, the marker may be detected by detection equipment at a store exit, thereby causing a false alarm.
Even when the marker signal is at the nominal frequency, the timing of the detection circuit is critical. If detection takes too long or if triggering is delayed, or if the marker is simply swept too rapidly, the deactivation signal field may be generated after the marker has passed through the region in which the deactivation field is radiated. Again, the outcome in such a case is a failure to deactivate the marker, and a potential false alarm at the store exit.